Examples of solid polymer electrolytes may include perfluoroalkylsulfonic acid, perfluoroalkylcarboxylic acid, sulfonated vinyl polymers such as polystyrene sulfonic acid and polymers obtainable by introducing a sulfonic or phosphoric group in a heat resistant polymer such as polybenzimidazole, polyether ketone and the like. Particularly, Nafion (Trade Mark) developed by Dupont Co., is known as a perfluoroalkylsulfonic acid type solid polymer electrolyte having excellent chemical stability. Further, a film made from the above solid polymer electrolyte is used for a proton exchange membrane of solid polymer electrolyte fuel cells.
It is said a proton transfer occurs in the presence of water. Therefore, when the solid polymer electrolyte membrane dries off, the specific resistance of the solid polymer electrolyte membrane becomes large and thereby the proton conductivity is lowered. Inversely, the more the solid polymer electrolyte membrane is moist, the smaller the specific resistance is. Therefore, it is possible to prepare a solid polymer electrolyte fuel cell having high performance. Further, a sufficiently moistened solid polymer electrolyte membrane prevents crossover such that hydrogen gas for feeding to an anode side and oxygen gas for feeding to a cathode side pass through the solid polymer electrolyte membrane as they are.
In order to improve the proton conductivity by inhibiting the fuel from crossover with keeping the solid polymer electrolyte membrane in a moisture state, it is very important to control the moisture of the solid polymer electrolyte membrane. As a conventional method of controlling the moisture of the solid polymer electrolyte membrane, disclosed are an indirect humidification method that hydrogen gas for feeding to an anode is saturated with water vapor to indirectly moisturize the solid polymer electrolyte membrane, and a direct humidification method that the solid polymer electrolyte membrane is made in a sandwich structure with sandwiching hydroscopic twisting-like fibers and the solid polymer electrolyte membrane is directly moisturized through the fibers (referred to JP-A-6(1994)-111827, JP-A-6(1994)-111834 and JP-A-7(1995)-90111).
The indirect humidification method, however, has problems such that it is difficult to control the water vapor pressure, and thereby the solid polymer electrolyte membrane is dried or the catalyst layer is moisturized too much to lower the performance. It further has defects such that operation with pressure is necessary because a large amount of water vapor is fed to hydrogen gas and the hydrogen gas is diluted and the partial pressure is lowered by the amount of the hydrogen gas diluted, and the performance is lowered because the diffusion of hydrogen gas in the electrode catalyst is inhibited.
As the countermeasure for the above problems, there is an attempt to thin the film thickness of the solid polymer electrolyte membrane for lowering the humidification amount with lowering the electric resistance of the membrane itself. The film thickness is decreased to cause a problem such that cross over of the fuel is easily induced and the cell voltage is lowered.
On the other hand, the direct humidification method has a problem such that because it has a sandwich structure with fibers sandwiched, the film thickness is increased and thereby the electric resistance is increased to lower the proton conductivity.
The present inventors have been studied on the problems associated with the prior art and earnestly studied in order to prepare a solid polymer electrolyte membrane having excellent proton conductivity and crossover inhibiting effect. They found that the proton conductive composition prepared by adding a metal catalyst to a polyarylene having a sulfonic group has capability of self-producing water, and due to the capability, the solid polymer electrolyte membrane is moisturized and thereby the proton conductivity thereof is improved and also the crossover can be inhibited.
They also found that when silica, the metal oxide fine particles and/or metal oxide fibers are added to polyarylene having a sulfonic group in addition to the above metal catalyst, the resulting proton conductive composition is improved on water-holding capability for holding water generated in the solid polymer electrolyte, and thereby drying in the solid polymer electrolyte is depressed, the proton conductivity is further improved and also the effect of inhibiting crossover is also further improved.